<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button id="btn" onclick="fun()">按钮</button>
  <script>
    function fun() {
      console.log('this', this)
    }
    // document.getElementById('btn').onclick = fun
  </script>
</body>

</html>